Method for producing composite elements and composite element

ABSTRACT

A method is proposed for producing composite elements which have at least one core and cover layers containing two fibres, with the following steps:
         coating of open-pored, semi-finished products comprising fibre materials with fine-grained, pulverulent, cross-linking resin, in such a manner that the open porosity remains,   forming of an arrangement of at least one open-pored core and two coated semi-finished products in a forming tool which is heated to the cross-linking temperature of the pulverulent resin.

BACKGROUND OF THE INVENTION

The invention relates to a method for producing composite elementsaccording to the preamble of the main claim and a composite elementcorresponding to the preamble of the independent claim.

Composite elements in a sandwich arrangement with at least one core andcover layers are known. The core material is thereby built up based onrigid foam, semi-rigid foam or flexible foam polyurethane and/orpolypropylene and/or expandable polystyrene (EPS) and/or styrenederivatives (maleic acid-anhydride-styrene-acrylic). In the case ofthermoplastic structures, cover layers made of compact films are bondedby the foamed core to form the sandwich element and, after heating thecomposite, are formed in forming tools so as to form three-dimensionalbodies, such as for example ready-made inside roof linings in automobileconstruction. The sandwich arrangement comprising core layers and coverlayers is produced either by multilayer extrusion or by lamination. Thedecorative cover layers are generally not fibre-reinforced but can alsobe produced by applying fibres of different types (natural fibres, glassfibres) in the presence of thermoplastic binders, generally in the formof powders by means of sintering.

When producing polyurethane sandwich structures by cold-forming, afoamed core made of rigid or semi-rigid foam polyurethane, which isthermally formable still to a certain degree, is provided on both sideswith cover layers made of glass fibre/thermoplastic. The cover layersare thereby applied almost exclusively today in the on-line method bysprinkling thermoplastic powders with simultaneous addition of cut glassfibres from a roving by means of a cutting tool onto a conveyor belt forthe lower cover layer and likewise onto the foamed core plate for theupper cover layer. This composite is subsequently sintered in acontinuous heating device, for example in a double belt heating press,bonded and subsequently formed directly in a forming tool, into whichsimultaneously a textile is also introduced as surface decoration, so asto form three-dimensional bodies, for example moulded inside rooflinings for automobiles. Subsequently, trimming takes place by means ofpunching, water jet cutting or methods of this type and the finishedproduct is obtained, which is suitable for sheeting. This product issuitable for normal temperature usage up to approximately 105° C.

In another embodiment of the polyurethane application, the high adhesiveforce of polyurethane mixtures is used for bonding with the coverlayers. A mixture of polyurethane polyol components with activators andMDI (44′ methylene di(phenylisocyanate) is thereby sprayed on a carriermaterial, for example polyethylene film, paper with polyethylene and/orpolyurethane film covering, and glass fibre pieces are applied to thelower and upper cover layers. The decorative covering is undertakengenerally by a textile with a sealing film and the prepared composite isimmediately formed in a forming tool preheated to approximately 50-80°C. and cured. The products obtained after trimming are also usable atincreased temperatures up to approximately 125° C.

The disadvantage of all these known composites is the inadequateacoustic effect since either only closed-pore core materials are used orthe cover layers are no longer permeable due to sealing measures as inthe just described embodiments. As a result, the absorbent effectdesired in the interior of for example automobiles for reducing airbornenoise is no longer offered.

For acoustic insulation parts in the engine compartment, such as forexample bonnets, there are used nowadays so-called light foams,polyurethane semi-rigid foams with low relative density of 6-20 kg/m³and surface mats based on polyethylene terephthalate(PET)/cellulose/polyacrylic (PA)/C-polyacrylonitrile (C-PAN) andmixtures of these. The surface mats are provided with adhesive coatingsbased on polyethylene (PE) (CoPES/MF (chemical fibres made ofpolyester-copolymers/melamine formaldehyde) and are processed in hottools at 140-200° C. to form formed parts. To date more rigid productshave been produced for use mainly by choice of surface mats. Thedisadvantage is an expensive cover mat with reduced acoustic behaviour.If surface mats with inserts made of glass fibres, which were producedby extrusion coating with polypropylene, are used the acousticproperties are reduced likewise. In addition structures of this type arenot stable enough at high temperatures greater than 100° C.

SUMMARY OF THE INVENTION

The object therefore underlying the invention is to produce a method forproducing rigid, light sandwich composite elements with supporting coverlayers and the composite elements themselves, which also remaindimensionally stable even at fairly high temperatures up to 150° C. andwhich are damping with respect to acoustics.

This object is achieved according to the invention by the methodaccording to the main claim and the composite element of the independentclaim.

As a result of the fact that an open-pored, semi-finished productcomprising fibre materials is coated with fine-grained, pulverulent,cross-linking resin in such a way that the open porosity remains and asandwich arrangement, comprising at least one open-pored core and twocoated semi-finished products forming the cover layers, is formed, whichis formed in a forming tool heated to the cross-linking temperature ofthe pulverulent resin so as to form the composite element, thiscomposite has a high acoustic and thermal insulating effect, with asimultaneous temperature-stable, rigid and light design and makespossible the use both in the interior of a vehicle and also forstructures in the engine compartment, such as end walls, bonnetabsorbers, transmission tunnels and the like.

The “open porosity” is ensured by two measures or properties of the usedmaterials.

-   -   1. The used powder materials are very fine-grained and are not        film-forming or only slightly.    -   2. The used fibre mats are fine-fibred and have a high specific        surface.

Due to this combination, a coating of up to 200% of the used fibre matcan be achieved without a closed or predominantly closed layer beingproduced. Coating of the fibres and bonding of the fibres takes placepredominantly at the intersection points. Consequently, the strength ofthe fibre mats is achieved and in particular the shearing strength(folding) is significantly improved.

Even during the production process of the composite elements, it remainsin the previously described state since melting and cross-linking of thepulverulent resins only takes place at the place of the attachment tothe fibre mat.

Due to the measures indicated in the sub-claims, advantageousdevelopments and improvements are possible.

Preferably, the cover layer is produced as coated semi-finished productsseparately from the actual core. For the semi-finished products,corresponding to the subsequent purpose of the composite element,different materials from the following group are selected alone or incombination with each other: glass fibre mats, glass fibre layering,continuous or staple fibre glass fibre mats, synthetic fibre mats madeof for example polyethylene, polyamide or others, as long as theirmelting point is higher than 160° C., natural fibre mats made of jute,hemp, sisal, flax, kenaf, cotton and/or metal fibres or metal fibreknitted materials or mixtures.

The fibre materials formed as semi-finished products are provided bymethods, which are basically known in the state of the art, with apowder matrix made of fine-grained, cross-linking, i.e. reactive resin,the reactive resin being bonded to the fibres by preheating at atemperature lying below the cross-linking temperature.

As reactive resin there can be used the most varied of materials, forexample ethylene-propylene-copolymers, unsaturated polyesters,polyurethanes and combinations thereof, leftover materials from thepowder coating industry based on polyester, polyamides, polyacrylates,powder products based on phenol resin and combinations of the previouslymentioned materials. These resins can be used as powder mixture with aheterogeneous composition or they are adjusted by addition of modifiersand accelerators, such as imidazole compounds, to the specificapplication case.

As already explained above, various methods for coating the fibrematerials with the powder matrix can be provided. In an advantageousmanner, the coating takes places according to the method of bonding andattaching by means of a film-forming medium. The pulverulent reactiveresin is thereby converted with a thickening agent into an aqueousdispersion. As thickening agent for producing an aqueous dispersionthere can be used for example cetylmethylcellulose, vinyl acetatedispersions, polyvinyl alcohol (PVA) solutions, starch, polysaccharidesand the like. After coating with the dispersion, the semi-finishedproducts are dried and in fact at temperatures at which the reactivepowder resins are not yet activated and a premature cross-linking isprevented. The coated semi-finished products are then available forfurther processing.

In the case of another coating method, the reactive powder resins aresprinkled on the semi-finished product and melted-on at correspondingtemperatures so that the resin combines with the fibres. For respectivecoating methods, a foaming agent can also be added. There can be appliedas coating methods, the methods known per se, such as painting,spreading, spraying, transferring, splashing, immersing, padding etc.

The coating quantity is based on the subsequent use and is normally20-200% of the surface weight in g/m² of the respective semi-finishedproduct made of fibre material.

A lower coating quantity produces a high air permeability and lowrigidity, whilst a high coating quantity produces a lower airpermeability and high rigidity.

Due to the type of fibres or fibre fleeces or mats which are used andthe choice of the reactive powder resin, a network is produced which hasa high proportion of open structures. The choice is thereby based on thesubsequent use, temperature stressing, flame resistance etc. Hence, theair permeability is ensured which is required for an effective soundabsorption. According to the fibre type, powder resin type and coatingquantity, the air permeability can be controlled such that a flowresistance, which is established for subsequent use and noise reduction,can be set. In a preferred manner, various flow resistances can beassigned hence to the cover layers of a composite element if it servesthe purpose of use.

As a material for the open-pored core there can be used for example afoam or fibre material which cannot melt at a temperature up to 200° C.and is adjustable in its compressive strength or resistance to pressure.The semi-rigid, thermally deformable polyurethane foams have provedthereby to be particularly suitable. There can be also used as corematerial however, a honeycomb structure produced from saturated orunsaturated paper and/or from perforated aluminium. There can be used asfurther core material also knitted fabrics produced by textiletechniques which are known under the term spaced weaves. Other materialsare also conceivable which, like the above-described materials, have noclosed pores and which are therefore suitable for sound absorption.

In an advantageous manner, the structure of the core material can bemodified for thermal insulation by insertion of hightemperature-resistant materials, such as mineral fibre mats, MF foams,silicate fibre papers or mats.

In order to produce the composite elements, the coated semi-finishedproducts as cover layers and the open-pored cores are placed in asandwich arrangement. The coated semi-finished products can be stored inthe form of rolls or blanks. During their processing into compositeelements, these coated semi-finished products are supplied to aprocessing plant for layer-wise construction. In this plant, therespective semi-finished product is sprayed with a predeterminedquantity of water by pure atomisation. The quantity is based upon thetype and deformation degree of the formed part and is produced via thequantity regulation known in the state of the art. Consequently, thepreviously fairly inflexible mat becomes wet through and flexible andcan be formed extensively in a form-shaping manner. In addition, thebonding of the powder particles produced by binders or thickening agentsis reduced and hence better wetting of the different layers of theconstruction is achieved.

For the forming and curing, the sandwich arrangement comprising corelayer and upper and lower cover layer is transported into a forming toolheated to cross-linking temperature of the respective resin. The formingtool must be provided with vents corresponding to the mould,conveniently on both mould halves in order that the residual moisturestill present during curing can escape from the forming tool without afairly large pressure build-up. For rapid heating of the composite,regulation of the pressure build-up and pressure reduction can beimplemented also and hence a shorter mould dwell time is achieved. Inorder to avoid damage to the composite and/or to the plant component,the pressure reduction must be effected before opening the forming toolsuch that in a predetermined time interval, the pressure is reduced toatmospheric pressure.

In a further forming method, the composite elements can also beprocessed without pre-moistening of the coated semi-finished products.For this purpose, the sandwich arrangement or the composite is heatedfirstly to a temperature which is above the melting temperature of therespectively used reactive resin and the composites heated in thismanner are transported directly and rapidly into the forming tool heatedto cross-linking temperature of the respective resin. In the case ofthis method also, vents should be provided in the forming tools.

The formed parts or composite elements removed from the forming toolscan then be supplied to their purpose of use or be provided in a secondoperating step with decorative surface layers. The composite elementscan be used as covering parts in the interior of vehicles, such as forexample ready-made inner roof linings, said parts having excellent heatresistance and acoustic properties. For acoustically effective parts inthe engine compartment, the composite elements can be used asdeflection-resistant, very light parts which are provided with fewattachment points because of their high intrinsic rigidity. Furthermore,the composite elements are provided as bonnets, motor-side end walls,tunnel insulations, engine compartment bulkheads, spare wheel cavitiesor insulations and tank insulations, both noise reduction and alsothermal protection being offered. For inflammable surroundings, thecover layers can also be provided with flame protection in the form ofsolid materials, such as aluminium hydroxide, melamine resin powder, sothat their inflammability is low.

Of course, a plurality of layers can also be provided in a correspondingmanner in various sequences for the composite elements, it is essentialhowever that the open-pored character of the entire arrangement remainseven after forming and after the effect of temperatures between 120-200°C.

BRIEF DESCRIPTION OF THE DRAWINGS

An example of a composite element is shown in the accompanying drawing,which is represented in one FIGURE.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

An example of a composite element is specified in the followingcorresponding to the single FIGURE. The sandwich construction for thecomposite element comprises thereby a core 1 in polyurethane light foamwith 12-15 kg/m³, two semi-finished products or cover layers 2 which arecoated and disposed on respectively one side of the core and cover mats3 applied thereon. This arrangement serves for producing strong anddeflection-resistant insulation parts. The cover layers comprise coatedglass mat, for example Microlit SAC 50/2 and the cover mat is formed onthe basis of viscose/PET with 45 g/m². The glass mat which is used inthe cover layer 2 is coated on a flat underlayer with a coating mass bymeans of rolling-on. The coating quantity is chosen such that at most220 g/m² of the coating is applied wet and distributed uniformly. Thecoated glass mat is subsequently dried in a drying device until themoisture falls below 6% by weight. Subsequently, the dried glass matscan be stored. In the normal conditions of 20-25° C./50-55% relativemoisture, the glass mats, packed in films, can be stored for severalweeks.

Should an injection-moulding method be chosen for the coating,application on both sides with respectively 110-120 g/m² is requiredbecause of the solid material content since the glass mat acts as a“filter”. During the injection-moulding method, care must be taken toprovide a uniform distribution and a wet-in-wet application. The dryingis implemented in the previously described manner.

In order to produce the composite according to the FIGURE, first of alla layer of cover mat with a surface weight of 30-50 g/m² and a PEsintering is placed with the sintered side upwards on a depositingtable. A layer of the coated glass mat is applied hereon. The glass matis moistened slightly by spraying with fine nozzles in the dried state.Only a light but uniform moistening should be effected hereby, generally50-80 g/m² water being sufficient. The polyurethane light foam corematerial is placed on the moistened glass mat. This is thereaftercovered with a layer of coated glass mat which is moistened in a similarmanner, the upper cover mat being placed with the PE side towards theglass mat. The thus prepared sandwich packet is introduced into aforming press.

The forming press is closed at a temperature of 170° C.±10° C. until alight contact with the packet is achieved. The packet is preheated for20 seconds and then the press is closed and pressed initially for 40seconds. Thereafter the press is opened and possibly any required covermat is placed therein according to choice. The press is closed onceagain for 20 seconds. After a total pressing time of 80 seconds, theformed part is placed on a form-fitting place of deposit in order tocool.

The relative airflow resistance of the composite elements according tothe invention is between 150 and 450 kNs/m⁴.

1. A composite element having at least one core and cover layerscontaining two fibers in a sandwich layer, wherein the core comprises anopen-pored material and the cover layers comprise fiber materialsprovided with a coating and present in the form of semi-finishedproducts, the coating having reactive resin, which is a powder matrix ofcross-linking ethylene-propylene copolymers, unsaturated polyestersand/or polyurethanes and cetyl-methylcellulose, vinylacetatedispersions, PVA solutions, starch or polysaccharides as a thickeningagent.
 2. A composite element according to claim 1, wherein thesemi-finished product is formed as one of: a glass fiber mat, glassfiber layering, continuous or staple fiber glass fiber mat, syntheticfiber mat made of PET, PA or the like with a melting point of higherthan 160° C., as a natural fiber mat made of jute, hemp, sisal, flax,kenaf, cotton or as metal fiber knitted material or mat.
 3. A compositeelement according to claim 1, wherein the core comprises an open-poredmaterial which cannot melt at a temperature up to 200° C.
 4. A compositeelement according to claim 3, wherein the core has embedded mineralfibers, MF foam and/or silicate fibers.
 5. A composite element accordingto claim 1, wherein a plurality of open-pored cores is provided withdifferent materials and properties with respective interposition of acoated semi-finished product.
 6. A composite element according to claim1, wherein it has a relative airflow resistance of 150-450 kNs/m⁴.
 7. Acomposite element according to claim 3, wherein the core comprises afoamed or fiber material, or honeycomb structure produced from beamed orunsaturated paper or perforated aluminum, or knitted materials producedby textile techniques.
 8. A composite element according to claim 5,wherein the cover layers have different flow resistances.